1. Field of the Invention
The subject invention relates to liquid crystal displays and more particularly to improved AC activation circuitry for an active matrix addressed liquid crystal display.
2. Description of Related Art
Presently, all active matrix addressed liquid crystal displays use AC activation. Present AC displays use one transistor per liquid crystal cell to switch a stored control voltage across the cell. Such structures achieve AC biasing of the liquid crystal cells by driving the pixel voltage symmetrically about a fixed potential which is applied to the top plate electrode. This AC drive technique limits the maximum applied RMS voltage to one-half of the available DC voltage range. Since the new information on each pixel appears immediately after that pixel is updated, it is impossible to switch the top plate potential in such a way that the effects are uniform for each pixel.
Limiting the applied RMS voltage to one-half the available DC voltage is disadvantageous because it restricts the liquid crystal effects which may be used with AC drive. For example, AC dynamic scattering liquid crystal materials cannot be driven by present drive structures. In addition, other scattering liquid crystal effects which incorporate encapsulated liquid crystal materials also require higher voltages than are presently available.
The long chain organic molecules of which liquid crystal materials are composed also can be degraded by various chemical contaminants. AC activated displays are more desirable than DC displays in this respect since the applied AC potential tends to limit electrochemical degradation of the liquid crystal material. It would therefore be desirable to continue use of AC activated displays, while eliminating the present limitations on the operating voltage of such displays.
It has occurred to the inventor that placement of additional transistors in the display cell structure might be used to eliminate the limitation on the operating voltage of the display cells in an AC activated display. Display cells with multiple transistors have been proposed in the prior art, generally to construct a static random access memory (RAM) cell. To the inventor's knowledge, additional transistors have not been proposed as part of a technique to extend the dynamic operating voltage of the liquid crystal cells.